1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate (e.g., a workpiece, an object, a display, etc.). The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, that is alternatively referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of a die, one die, or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means can comprise an array of individually controllable elements that generate the circuit pattern. Lithographic systems utilizing such arrays are generally described as maskless systems.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in that each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in that each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
The user of such apparatus defines a requested pattern to be formed on the substrate that is characterized by a particular dose-map of radiation. In maskless systems, data representing a requested dose-map is processed with reference to calibration measurements in order to obtain an actuating signal to send to the array of individually controllable elements. Areas of high pattern density in the dose-map require exposed spots on the substrate to overlap with each other. It has been found that, under certain circumstances, these regions of overlapping patterns can cause errors in the processing of the requested dose-map.
The process of joining one region of pattern to another region of pattern is known as seaming. Seaming can be carried out for patterns originating from a single optical column (e.g., receiving input from a single array of individually controllable elements), or between patterns from different optical columns (e.g., each receiving input from a separate array of individually controllable elements). Seaming in these situations can be achieved by arranging for the dose-maps of each of the overlapping patterns to reduce gradually over an overlap region. However, this approach can be difficult to regulate, particularly where the overlap region is narrow.
Therefore, what is needed is a lithography apparatus and device manufacturing method that improves the handling of regions prone to excess exposed spot overlap.